Sense-Data

Sense data are the alleged mind-dependent objects that we are
directly aware of in perception, and that have exactly the properties
they appear to have. For instance, sense data theorists say that,
upon viewing a tomato in normal conditions, one forms an image of the
tomato in one's mind. This image is red and round. The mental image
is an example of a “sense datum.” Many philosophers have
rejected the notion of sense data, either because they believe that
perception gives us direct awareness of physical phenomena,
rather than mere mental images, or because they believe that the
mental phenomena involved in perception do not have the
properties that appear to us (for instance, I might have a visual
experience representing a red, round tomato, but my experience is not
itself red or round). Defenders of sense data have argued, among
other things, that sense data are required to explain such phenomena
as perspectival variation, illusion, and hallucination. Critics of
sense data have objected to the theory's commitment to mind-body
dualism, the problems it raises for our knowledge of the external
world, its difficulty in locating sense data in physical space, and
its apparent commitment to the existence of objects with
indeterminate properties.

On the most common conception, sense data (singular:
“sense datum”)
have three defining characteristics:

Sense data are the kind of thing we are directly aware of in perception,

Sense data are dependent on the mind, and

Sense data have the properties that perceptually appear to us.

Each of those conditions calls for clarification.

First, condition (i): Everyone in the philosophy of perception agrees
that perception makes us aware of something. Most hold that there is
a distinction between the things perception makes us
directly aware of, and the things it makes us
indirectly aware of, where to be indirectly aware of
something is, roughly, to be aware of it in a way that depends on the
awareness of something else. There are at least two ways of further
explicating this notion. One way, adopted by Jackson (1977, pp.
15–20), is to say that we perceive something indirectly if we
perceive it by virtue of perceiving something else. For
example, consider my perception of the table in front of me. I do not
perceive all of the table; I can only see its outer surface, and
indeed only the portion of that surface facing me. Yet we still say
that I see the table. I count as seeing the table by virtue
of seeing something else, namely, the facing surface of the
table. Therefore, Jackson would say that I see the table only
indirectly. Sense data, on this view, are things that one may
perceive not by virtue of perceiving anything else.

Another way of distinguishing direct and indirect awareness is to
say, roughly, that one has indirect awareness of x when
one's awareness of x is caused by one's awareness of
something else (see Huemer [2001, pp. 55–7] for a more spelled-out
version of this approach). For instance, I might wish to determine
the temperature of a pot of water indirectly, by means of a
thermometer, rather than sticking my hand in the water. In such a
case, I would first become aware of the reading on the thermometer,
and this would cause me to be aware of the temperature of the water.
Thus, my awareness of the water's temperature would be indirect. On
this view, sense data would be things our awareness of which is not
causally dependent on any other awareness.

Second, condition (ii): Sense data theorists believe that the things
we are directly aware of in perception are dependent on the mind of
the perceiver — they cannot exist unperceived. Such things have
also sometimes been called “mental images,”
“ideas,” “impressions,”
“appearances,” or “percepts.”

Third, condition (iii): “The properties that perceptually
appear to us” refers to the qualities we seem to perceive
things around us to have. For instance, if I perceive a tomato, and
it looks red and round to me, then redness and roundness are
properties that perceptually appear to me. According to those who
believe in sense data, there is in this case a thing that I am
directly aware of, which is both red and round, and which
depends on my mind for its existence. Condition (iii) holds true even
if I am subject to a sensory illusion or hallucination. Thus, if the
tomato is really green, but, due to some sort of color illusion, it
looks red to me, then my sense datum is red, not green.
Furthermore, if there is no tomato present in reality, but I am
hallucinating a tomato, then I will be having a tomato-like sense
datum.

Those who accept sense data believe that sense data exist whenever a
person perceives anything, by any of the senses, and also whenever a
person has an experience qualitatively like perceiving, such
as a hallucination.

So construed, the sense data theory contrasts with two competing
schools of thought in the philosophy of perception. First, direct
realism holds that in perception, we are directly aware of
physical phenomena and only physical phenomena — for example, a
table, or a portion of a table's surface. Direct realists thus deny
that there is anything satisfying both conditions (i) and (ii) above,
and therefore deny that there are sense data. Direct realism itself
comes in at least two varieties: a disjunctivist variety
(McDowell 1994; Dancy 1995) and an intentionalist variety
(Armstrong 1961; Searle 1983; Huemer 2001).

Second, the adverbial theory, in one version, holds that in
perception, we are directly aware of a certain kind of mental state
or occurrence, but that this mental state does not actually possess
the properties that appear to us (Chisholm 1977, pp. 29–30).
Adverbialists have been known to characterize this mental state in
such terms as “being appeared to redly.” When a person is
in the mental state of being appeared to redly, say the
adverbialists, it does not follow that anything is actually
red. Thus, adverbialists deny that there is anything
satisfying all of conditions (i), (ii), and (iii), and therefore deny
that there are sense data.

Those who accept sense data believe that sense data exist whenever a
person perceives anything, by any of the senses, and also whenever a
person has an experience qualitatively like perceiving, such
as a hallucination.

The term “sense data” has not always been used in the
sense described above. Indeed, when the term was first introduced by
early 20th-century philosophers such as H. H. Price, G. E.
Moore, and Bertrand Russell, it was intended only to denote that which
we are directly aware of in perception. The term's meaning was
supposed to be neutral between direct and indirect realist theories of
perception, so that it was not to be assumed either that sense data
must by definition be mind-dependent or that they must be
mind-independent (Russell 1912 [1997], p. 12; Moore 1953,
p. 30). Thus, G.E. Moore debated with himself, inconclusively, about
whether “sense data” were or were not typically parts of
the surfaces of physical objects. Broad (1925) thought sense data were
neither mental nor physical. And more recently, Bermúdez (2000)
has defended what he calls a sense data theory, according to which the
surfaces of visually perceived physical objects are “sense
data.”

However, those who have used the term “sense data” have
so frequently concluded that what we are directly aware of in
perception is, in fact, dependent on the mind that the term is now
usually understood to import an assumption of mind-dependence.

Sense data theorists have also differed over exactly how to describe
the mind's relation to its sense data. Most sense data theorists have
said that we perceive sense data or, in the case of visual
sense data, that we literally see them (Jackson 1977; Ayer
1958; O'Shaughnessy 2003). Others say only that we are aware
of, are acquainted with, or simply sense sense
data (Robinson 1994; Price 1950, pp. 3–4), perhaps with the thought
that the terms “perceive”, “see,” and the
like should be reserved for our relation to the physical objects that
cause our sense data.

In the remainder of this article, sense data are understood in the
sense articulated in section 1.1, and the Sense Data Theory is
understood simply as the theory that there are such things, that is,
that in perception one is directly aware of mind-dependent things
that have the properties that perceptually appear to us.

The sense data theory was very popular, perhaps the orthodox view in
the philosophy of perception, in the early twentieth century. The
theory was advanced by Russell (1912); Broad (1925); Price (1950);
and Ayer (1956). Moore (1953, pp. 40–44) described the theory as
“the accepted view,” though he neither endorsed nor
rejected the theory himself. (Though Moore uses the term “sense
data,” he does not suppose that what he calls “sense
data” must be mental.) Since the mid-twentieth century, the
view's popularity greatly declined, though several philosophers have
continued to defend it (Jackson 1977; Robinson 1994; Casullo 1987;
Garcia-Carpintero 2001; O'Shaughnessy 2003).

Why has this theory been popular? A variety of arguments have been
given for recognizing sense data:

Perspectival variation is the kind of variation in one's sensory
experiences that normally attends changes in one's spatial or other
physical relationship to the physical objects one is observing.
Perspectival variation, in this sense, is ubiquitous. For
instance, suppose you are viewing a table. If you move closer to or
farther from the table, your sensory experience changes. If you move
laterally relative to the table, your sensory experience will change
in another way (Russell 1912 [1997], pp. 8–11). In a famous
passage, Hume sought to use this phenomenon to show that what we are
immediately aware of in perception cannot be the real, external
objects, but must instead be only images in the mind:

The table which we see seems to diminish as we remove farther
from it. But the real table, which exists independent of us, suffers no
alteration. It was, therefore, nothing but its image which was present to the
mind. These are the obvious dictates of reason, and no man who reflects ever
doubted that the existences which we consider when we say this house and
that tree are nothing but perceptions in the mind, and fleeting copies
or representations of other existences which remain uniform and independent.
(Hume 1758, section XII.1; emphasis Hume's; punctuation has been modernized)

This argument and others like it are commonly characterized as
versions of “the argument from illusion,” though that
label can be misleading, since the phenomenon Hume appeals to in the
passage above is one of perspectival variation rather than
illusion. In the present article, I distinguish the argument
from perspectival variation from the argument from illusion proper;
illusions will be discussed in the following section.

Though Hume does not use the term “sense data,” the
mental images for which he contends are what 20th-century
thinkers labeled “sense data.” Here is one way of
understanding Hume's argument:

In the phenomenon of perspectival variation, the thing we are
directly aware of appears to change — for instance, its apparent
size or shape changes.

The real, external object does not change at this time.

Therefore, the thing we are directly aware of is not the real,
external object.

Once we have agreed that the immediate object of awareness is not the
real, external object, we are then meant to infer that it must be
some sort of image of the physical object in our minds, which we
perhaps confused with the physical object.

As Reid (1983, pp. 178–9) observes, the argument from (1) and (2) to
(3) is invalid, since the first premise speaks of apparent change,
whereas the second premise concerns actual change. There is no
contradiction in maintaining that the external object
appears to change but does not in fact change.

The argument could be made logically valid by rendering it like so:

In the phenomenon of perspectival variation, the thing we are
directly aware of changes.

The real, external object does not change at this time.

Therefore, the thing we are directly aware of is not the real,
external object.

Now critics of the argument will charge that premise (4) is false or
question-begging (Austin 1962, p. 30; Jackson 1977, pp. 107-8; Huemer
2001, p. 125) — it is at least as natural to say that the thing
we are directly aware of (namely, the physical object) merely appears
to change without actually changing. Perhaps this is mistaken, but
Hume has given no independent reason for rejecting this initially
plausible description of the situation. Sense data theorists have
often held it to be intuitively obvious that when we are directly,
perceptually aware of something, that thing must have the properties
that it appears to have (Price 1950, p. 3; Robinson 1994, p. 32;
Martin 2000, pp. 218–19). Opponents of sense data have typically
found this assumption unmotivated; J. L. Austin, the best-known
critic of the argument from illusion and related arguments for sense
data, is a case in point:

If, to take a rather different case, a church were
cunningly camouflaged so that it looked like a barn, how could any
serious question be raised about what we see when we look at it? We
see, of course, a church that now looks like a
barn. We do not see an immaterial barn, an immaterial
church, or an immaterial anything else. (1962, p. 30; emphasis
Austin's)

Austin's point seems to be that, just as a church can merely look
like a barn without there being anything that is a
barn, the table that we see in Hume's example may merely
seem to get smaller, without there being anything that
actually gets smaller.

Here is an alternate way of making out the argument from perspectival
variation:

An experience counts as awareness of x only if the
properties of the experience covary with certain of the properties of
x, so that when x changes, the experience changes,
and when x does not change, the experience does not
change.

In the phenomenon of perspectival variation, our sensory
experience changes, but the real, external object does not
change.

Therefore, our sensory experience does not count as awareness of
the real, external object.

Modifications might be made to this argument to make it more
plausible: the first premise might be put in counterfactual terms,
rather than in terms of actual changes; “direct
awareness” might replace “awareness”; and one might
specify more carefully in what respects the properties of experience
must covary with those of the object of awareness. Something like
this argument may be what Hume had in mind, if only implicitly.

Critics of this version of the argument may question either premise.
Thomas Reid seems to deny premise (8), arguing that the external
object changes in respect of certain relational properties. For
instance, when one moves farther away from a table, the table's
angular size relative to one's own position decreases, where
this is the size of the angle created by connecting the extremities of
the table to the point in space from which the table is viewed.
Though this property is relational, the relationship involved is a
purely physical one, holding between physical things such as the table
and the eye, so it might be said that there is no need to introduce
mind-dependent sense data as objects of awareness (Reid 1983,
pp. 176–8; Huemer 2001, pp. 120–23; Cornman 1975,
pp. 58–9).

The Argument from Illusion is the best-known and most historically
influential argument for the existence of sense data. An illusion is a
case in which one perceives an object, but the object is not the way
it appears in some respects. For instance, when one views a straight
stick half-submerged in water, the stick may appear bent. Since it is
not in fact bent, this is an illusion. Some philosophers have argued
that the possibility of such sensory illusions shows that what we are
directly aware of in perception is never the real, physical object
(Ayer 1963, pp. 3–11). Using the bent-stick illusion as an
example, one might argue:

When viewing a straight stick half-submerged in water, one is
directly aware of something bent.

No relevant physical thing is bent in this situation.

Therefore, in this situation, one is directly aware of something
non-physical.

What one is directly aware of in this situation is the same kind
of thing that one is directly aware of in normal,
non-illusory perception.

Therefore, in normal perception, one is directly aware of
non-physical things.

A background assumption is that there is only one stick-like thing
that one sees in the example, and that thing is either an actual,
physical stick, or a sense datum of a stick. The argument concludes
that it is not the physical stick, so it must be a sense datum.

Step (4) seems plausible, since one can imagine first perceiving the
stick normally, and then moving it into the water. It would be
implausible to maintain that one is seeing the physical stick up to
the moment when it touches the water, at which point the object of
one's awareness suddenly changes to a sense datum.

Opponents of sense data object to premise (1) on grounds similar to
those considered in section 2.1: namely, it may be that what one is
directly aware of merely appears bent but is not in fact bent. Sense
data theorists and their opponents, again, disagree over whether an
object of direct awareness must have exactly the features it appears
to have.

Having been taken to task by Austin (1962) over the argument from
illusion, A. J. Ayer sought to defend sense data by another argument
(though Ayer seems to think it is the same argument):

What the argument from illusion . . .
does clearly establish is . . . that there is not a perfect coincidence between
appearance and reality. It shows that if we were always to take appearances as
it were at their face value we should sometimes go wrong and, what is important
here, that we should go wrong predictively. When we misidentify an object, or
misjudge its properties, or misperceive its status, taking it for example to be
a physical solid when it is in fact an image, we issue a draft on our further
experiences which they fail to honour. But this again implies that our
judgements of perception are, in my sense, inferential. (Ayer 1967, p. 129)

By “judgements of perception,” Ayer means beliefs about
the physical world that express what we seem to perceive to be the
case; for instance, when I see a chair, I normally make the
“perceptual judgement” that a physical chair is present.
Ayer's central premise seems to be that all such beliefs about the
physical world are fallible; somehow, this is supposed to force the
conclusion that such beliefs are inferential. That, in turn, is
supposed to support the sense data theory.

Perhaps Ayer's implicit reasoning is this:

If
one is directly aware of something, then one can have non-inferential
knowledge of facts about it. (Premise.)

If
one knows non-inferentially that p, then one's belief that p
is infallible. (Premise.)

No
belief about the physical world can be infallible. (Established by the
possibility of illusion, hallucination, etc.)

Therefore,
no one can have non-inferential knowledge about the physical world. (From
2, 3.)

Therefore,
no one is directly aware of anything physical. (From 1, 4.)

Conclusion (5) does not suffice to establish the existence of sense
data, but by ruling out the competing direct realist theory, it would
take Ayer a considerable distance towards vindicating the sense data
theory. If beliefs about sense data could plausibly be claimed to be
infallible, and if one assumes a foundationalist epistemology,
beliefs about sense data would be prime candidates for constituting
non-inferential knowledge. This would make sense data very plausible
candidates for objects of direct awareness.

A hallucination is a case in which one has an experience
qualitatively like perception, but there is no external object that
one is perceiving. For instance, a large dose of LSD might cause me
to have an experience of seeming to see a pink rat on this table,
where there is in reality nothing pink-rat-like.

Some believe that the possibility of hallucinations shows that even
normal perception always involves sense data (Robinson 1994, pp.
151–62; Jackson 1977, pp. 50ff.). Imagine two people, Sally and Sam,
each of whom is having an experience of seeming to see a pineapple.
Sally is simply perceiving a pineapple in the normal way. Sam,
however, is having an incredibly realistic hallucination of a
pineapple, induced by brain scientists who have sophisticated
technology for electrically stimulating Sam's brain. And suppose, as
is theoretically possible, that the brain state causally relevant to
Sally's visual experience is the same as the brain state causally
relevant to Sam's visual experience. I will call this brain state B.
Sam would be unable to distinguish his experience from a normal
perception of a pineapple.

In this scenario, what is Sam directly aware of? Surely not a
physical pineapple, since no physical pineapple is present. It seems,
then, that he must be aware of a mere mental image of a pineapple.
This mental image is caused by brain state B.

Now, what about Sally? Sally's brain state was caused in a different
way from Sam's — Sally's was caused by a real pineapple,
whereas Sam's was caused by the brain scientists. But that does not
change the fact that Sally is now in the same brain state as Sam. We
have already said that in Sam, brain state B caused a mental image of
a pineapple. Therefore, it seems that if someone else were to have
state B, it would also cause a mental image of a pineapple for them.
Therefore, it seems that Sally must also be having a mental image of
a pineapple, since she is in state B. Therefore, normal perception
involves sense data, just as hallucination does. This argument relies
on the principle that, if a causal chain of events leads to some
effect, E, then any series of events that duplicates the last
member of the causal chain will also produce E, regardless of
whether the earlier members of the chain are duplicated. As long as
Sally and Sam get into the same brain state, regardless of how they
got there, both should experience whatever effects result from that
brain state.

One way for a critic of sense data to respond to this argument would
be to deny that state B causes Sam to have a mental object of
awareness. According to the intentionalist account of
perception, what Sam has is a mental state that falsely represents
there to be a pineapple. Sally also has a mental state that
represents there to be a pineapple, though in her case the
representation is true. It may be held that Sam's mental state has
no object of awareness since it is entirely false, whereas
Sally's mental state has the physical pineapple as its object of
awareness. Thus, in neither case must we posit a mental object of
awareness, as in the sense data theory (Huemer 2001, pp. 127–8).

Hume tells us that one can induce a case of double vision in oneself
by merely pushing on one eye with one's finger. The possibility of
double vision, he believes, shows that the immediate objects of
awareness in perception are not the real, physical objects (Hume
1739, I.IV.ii; see also Broad 1925, pp. 187–8). The intended argument
may be something like this:

In a case of double vision, one sees two of
something.

There are not two (relevant) physical objects
in this situation.

Therefore, in a case of double vision, one
sees something non-physical.

It would be implausible to maintain that one of the two things is a
sense datum while the other is a real object. One reason this would
be implausible is that there seems to be nothing relevantly different
between the two things that could make one of them the
“real” object. Therefore, one should conclude that both
of the things one sees are sense data, rather than physical objects.

Critics might respond to this argument by claiming that in a case of
double vision, rather than seeing two things, one sees a single thing
that merely appears to be in each of two places (Huemer
2001, pp. 130–31).

There is always a time delay between any event in the physical world
and our perception of it. This is most stark in the case of distant
stars, which may burn out and yet still be “seen”
thousands of years later, as the light continues to travel the
distance between the star and us.

Imagine two individuals, Sally and Sam, who are each looking up at
the night sky and “seeing” — or seeming to see
— qualitatively similar stars. The star responsible for Sally's
experience still exists. But the star ultimately responsible for
Sam's experience ceased to exist 1000 years ago. Sam still
“sees” it because the star was over 1000 light years
away.

What is Sam directly aware of? Surely not an actual star, since no
star presently exists in the place where he is looking. It must be a
mere mental image of a star that he is directly aware of. Just as in
the case of the argument from hallucination, we can now argue that
since Sally is in the same brain state as Sam, she must also be having
a mental image of a star. Therefore, sense data are involved in normal
perception, even when the physical object responsible for the
perception still exists. (Russell 1912 [1997], p. 33; Robinson 1994,
pp. 80–84. Ayer [1956, pp. 102–4] discusses the argument
without endorsing it.)

One might be tempted to say that what Sam sees is light
rays, rather than a sense datum. But if the time gap shows that
Sam does not directly perceive the star, it must also show that Sam
does not directly perceive anything else outside of him either, since
there is some time delay, however small, between any
external event and Sam's corresponding sensory experience. Sam's
visual experience as of a star will occur at least slightly later
than the light rays strike his retina.

The natural reply for theorists wishing to resist sense data is to
claim that one can “see into the past,” that is, that
one's perceptual experiences may represent past states of affairs, or
represent objects as they were at an earlier time (Cornman 1975, pp.
49–50; Huemer 2001, pp. 131–5).

Many philosophers have held that the so-called “secondary
qualities” — including such qualities as colors, tastes,
smells, and sounds — do not exist in the external world, and
that we must instead recognize them as properties of sense data.
Consider the case of colors. A sense data theorist might argue:

Everything we directly see has color.

No physical thing is colored.

Therefore, everything we directly see is
non-physical.

(See Russell 1912 [1997], pp. 8–11; Jackson 1977,
pp. 120–37; Robinson 1994, pp. 59–74.) The first premise
seems obvious on its face. The second premise may seem unbelievable,
but there are several arguments for it.

One of these arguments appeals to differences of color perception
among people. Not only color blind people, but even people with
normal vision differ among themselves slightly in how they perceive
the colors of things (Hardin 1988, pp. 79–80; Byrne and Hilbert 1997,
pp. 272–4). If colors are really out there, then there would have to
be an answer to the question, Whose color perceptions are right? But
not only is there no way of determining the answer to that; it seems
hard to think of what facts might make one person's color perceptions
more correct than another's. A related argument appeals to the
differences of color perception among different species of animals
(on these differences, see Jacobs 1981, chapter 5; Varela et al.
1993). Again, there seems no answer to the question of which species
is right.

Another argument appeals to the fact that our experiences of color
are caused by the wavelengths of light that physical objects reflect.
Therefore, it seems that if colors belong to physical objects, they
must be reducible to spectral reflectance distributions (as Byrne and
Hilbert [1997] claim). However, there is in general no single
spectral reflectance distribution, or even a single continuous range
of spectral reflectance distributions, corresponding to each of the
colors we see. Two objects with very different spectral reflectance
distributions may both appear orange to us in normal lighting
conditions, for example. (This phenomenon is known as
“metamerism.”) Some believe that this fact precludes our
reducing colors to spectral reflectance properties (Hardin 1988, pp.
7, 46–8).

Some philosophers hold that colors are dispositions to cause certain
kinds of sensory experiences in us, rather than dispositions to
reflect light in certain ways. But others object that this is not so,
because colors ought to be properties that we directly perceive
things to have, whereas we do not perceive things as having
dispositions to cause experiences in us.

There is a good deal to be said about color, and a good deal yet to
be resolved. The ultimate acceptability of premise (2) of the above
argument will turn on whether some reductive theory of the nature of
colors is defensible.

One reason the sense data theory has lost favor is no doubt the
ascendance of physicalism in the philosophy of mind.
Physicalists believe that the world is entirely physical; in
particular, they believe that mental states either do not exist or
are reducible to physical states, such as brain states. Physicalism
is contrasted with dualism, which holds that mental
states/events are distinct from physical states/events.

For various reasons, most contemporary thinkers in philosophy of mind
embrace some form of physicalism and reject dualism. If they are
right to do so, then there is a reason for rejecting sense data:
namely, that sense data do not seem to fit into the physicalist
picture (Martin [2000, p. 222] discusses but does not endorse this
line of thought).

Sense data are supposed to have the properties that perceptually
appear to us. But, in cases of normal perception, the only
physical things that have the properties that perceptually
appear to us are the external objects that the direct realists say we
are perceiving; and in cases of illusions and hallucinations, there
are no physical things that have the properties that perceptually
appear to us. In particular, our brain states manifestly do not
ordinarily have the properties that perceptually appear to us (except
in the odd case that we happen to be looking at a brain). So sense
data, if they exist, must be non-physical things.

O'Shaughnessy (2003, p. 186) seeks to avoid this consequence by
distinguishing the place where a sense-datum is from the
place where it “is given experientially to us”.
Presumably he would draw a similar distinction for other properties
of the sense-datum. His view seems to be that sense data might be
identical with brain states, so that the sense data one experiences
would in fact have the properties, such as shape, location, and
perhaps color, of one's brain states, even though they are given
experientially as having different and incompatible sets of
properties. O'Shaughnessy does not explain what it is for a thing to
be given experientially as having a property, but he seems to be
abandoning the traditional doctrine that sense data literally have
the features that perceptually appear to us.

A more perspicuous response to the argument from physicalism is to
simply embrace mind-body dualism (Jackson 1982).

At least three sorts of epistemological objections to the sense data
theory have been raised. The first and most common charge is that the
sense data theory leaves us vulnerable to external world skepticism.
If we are only ever directly aware of our own sense data and other
nonphysical phenomena, it is said, then it is unclear what reason we
have for believing anything physical exists. Sense data theorists
will generally admit that it is logically possible that someone
should have exactly the same sense data that I, for example, have,
and yet for there to be no physical objects around that person of the
kind that I take myself to be surrounded by. Berkeley (1710, section
20) famously took this point to show that I have no good reason for
believing in such physical objects. However, as Jackson (1977, pp.
141–2) observes, the point really shows only that we cannot validly
deduce the existence of physical things from facts about our
sense data; it remains open that we might infer the existence of
physical things non-demonstratively. To rule this out, one may appeal
to Hume's (1758, XII.1) skeptical argument, according to which all
non-demonstrative reasoning proceeds by induction, and all inductive
reasoning consists in generalizing from past experience. On this
view, in order to non-demonstratively infer any conclusion about
physical objects, one must first have past experience of physical
objects from which one might draw generalizations. If, as the sense
data theory holds, one's immediate experience only ever concerns
sense data, then one's inductive inferences can only draw
generalizations about sense data.

Sense data theorists can respond to this skeptical challenge by
proposing that our beliefs about the physical world are justified by
inference to the best explanation (Jackson 1977, pp. 142–5;
Russell 1912 [1997], pp. 22–4). Consider an analogy: we know of
the existence of molecules, despite never having directly observed a
molecule, because the theory that posits molecules provides the best
explanation for certain other things we know about the behavior of
macroscopic bodies. Similarly, perhaps we know of the existence of
physical objects in general, despite never having directly observed
one, because the theory that posits physical objects provides the best
explanation for other things that we know about the behavior of sense
data.

A second broadly epistemological objection claims that the sense data
theorist cannot account for our having the concept of physical
objects, or for our ability to conceive of the properties of physical
objects. This is because, according to the sense data theory,
physical objects in principle cannot be directly observed in the way
sense data can. Thus, while a sense datum may, for example, be red
and round, all physical objects are invisible (they cannot
be seen). It makes no sense to say that a color resembles something
that is invisible, and similar arguments could be made for all other
observable properties besides color; therefore, physical objects
cannot in principle resemble sense data. Since we are supposedly
never directly aware of physical objects or their properties, and
they cannot resemble the things we are directly aware of, it is
argued that we could have no conception of the nature of physical
objects (Berkeley 1710, sections 8–10; Searle 1983, pp. 59–60).

Sense data theorists will reply first by denying that on their view
physical objects are “invisible.” Rather, their view is
that what it is to see a physical object is to have a sense
datum representing that object, so physical objects are, on their
view, often seen (O'Shaughnessy 2003, pp. 175, 178–9). Second, the
objection of the preceding paragraph gains undeserved plausibility
from the use of the word “resemble.” The statement that A
resembles B may be taken to mean that A looks like B. Sense
data theorists are not committed to claiming that sense data look
like physical objects. They are, however, committed to claiming
that sense data have at least some of the properties that
physical objects typically have. In particular, most sense data
theorists will agree that physical objects, like sense data, have
shapes, though they will typically deny that physical objects have
colors or other secondary qualities (Locke 1689, II.viii; Jackson
1977, 120–37). Pace Berkeley, it is not unintelligible to speak of an
object one is directly aware of having the same shape as an object
one cannot be directly aware of. No one thinks, for example, that
because an individual H2O molecule cannot be seen, it is
therefore unintelligible to speak of the molecule's shape.

A third epistemological objection derives from Wilfrid Sellars
(2000), who questions the traditional account of foundational
empirical knowledge (knowledge that comes immediately from
experience). The epistemological view traditionally taken by sense
data theorists has been roughly along these lines (Russell 1912):

First, one has a sense datum.

When one has a sense datum, one is necessarily immediately and
infallibly aware of that sense datum. This immediate awareness is
known as “sensing” or “being acquainted with”
the sense datum.

By virtue of this acquaintance, one is in a position to know
that one has a sense datum of the kind that one in fact has.

One then makes inferences about the physical world to explain
the series of sense data that one has.

The first epistemological objection discussed above questions step
(d). Sellars, however, questions step (c). He poses a dilemma for
sense data theorists: either the immediate awareness of a sense datum
mentioned in (b) and (c) is propositional in form (that is,
it is the awareness that the sense datum has F, where F is some
property), or it is non-propositional. If the awareness is
propositional, says Sellars, then it requires the application of
concepts. For instance, to be aware that a sense datum is red, one
must first have the concept of redness. This is problematic,
because it is generally thought that perceptual awareness ought to
precede and be independent of concepts. On the other hand, if the
awareness in step (b) is non-propositional, then it cannot give one
the knowledge posited in step (c), because that knowledge is
propositional — it involves the knowledge that one's sense
datum is of a certain kind — and a non-propositional state
cannot support a proposition (Sellars 2000, part I).

One reply on behalf of the sense data theorist is to note that
Sellars' dilemma is not particularly directed at the sense data
theory, despite that Sellars formulates it in those terms. That is,
if Sellars' argument is compelling, a version of it would apply
equally well to direct realist, idealist, or adverbial theories of
perception. Sellars' real objection is to the idea of any
form of direct awareness providing us with knowledge, whether it be
awareness of sense data, physical objects, states of being appeared
to, or anything else. Sellars' intended solution to the problem seems
to lie in the direction of a coherence theory of justification. But
it is unclear why a sense data theorist could not equally appeal to
considerations of coherence, despite that historically all or most
sense data theorists have in fact been foundationalists.

A second reply, on behalf of the sense data foundationalist, is that
Sellars has confused propositional awareness with conceptual
awareness. One might enjoy an immediate awareness of a sense datum as
having a certain specific shade of color for which one has no
preexisting concept. The awareness would thus be non-conceptual but
propositional: one is aware of the fact that a is
F, where a is the sense datum and F is the
unconceptualized property one senses it as having (Huemer [2001, pp.
71–7] takes a similar line, but adapted to a direct realist view).

If sense data have the properties that perceptually appear to us,
then among other things, visual sense data have sizes and shapes. If
so, then they occupy space. It is therefore fair to ask where in
space they are located. But there does not seem to be any plausible
answer to this (Huemer 2001, pp. 149–68).

One might propose that one's sense data are
literally inside one's head. This view would probably seem plausible only
if one identified sense data with brain states (as Russell [1927, p. 383]
and O'Shaughnessy [2003, p. 186] do). But this is problematic since one's
brain states do not generally have the properties that perceptually appear
to one. The brain state involved in seeing a table, for example, is not
table-shaped. Therefore, if one's sense datum is table-shaped, then the
sense datum is not the brain state.

One might propose that sense data are located
wherever the physical objects causing them are. Thus, when I look at a
table, my sense datum of a table is located right where the table is. But
this view would have trouble assimilating the sense data supposedly
involved in hallucinations. For this reason, the sense data theorist could
probably be pushed to the following view.

One might propose that sense data are located
wherever they appear to be (this appears to be Jackson's view
[1977, pp. 77–8, 102–3]). One problem with this view concerns experiences
of non-existent locations. For instance, one might have a vivid dream
about a fictional locale. If sense data are involved in illusions and
hallucinations, then presumably something like them is also involved in
dreams. But in this case, since the dreamt of place does not exist, one
cannot say that the sense data are located there.

A further objection to both answers (2) and (3) is that they conflict
with the special theory of relativity, since in some cases, they
would require one's brain state to cause a sense datum to appear
outside one's forward light cone, and the theory of relativity
precludes causal relations with events so situated.

Unable to find any plausible location for sense
data in physical space, some philosophers have proposed that sense data
occupy their own, separate space, sometimes called “phenomenal space”
(Broad 1925, p. 181; Russell 1927, pp. 252–3; Price 1950, pp. 246–52;
Smythies 2003). This view raises questions about how events in physical
space can interact with those in phenomenal space, and it also conflicts
with the theory of special relativity, which precludes the kind of
separation between space and time that the doctrine of phenomenal space
requires.

As we have noted, sense data are supposed to have precisely the
properties that are presented to us in perceptual experience. If one
has an experience of seeming to see something red, then one's sense
datum is red; equally importantly, if one is not having an
experience of seeming to see something red, then one does
not have a red sense datum.

A problem with this is raised by the observation that it is sometimes
indeterminate what properties objects appear to us to have. To say
that it is indeterminate what properties an object appears to have is
to say that the object appears to instantiate some
determinable, but there is no specific determinate
falling under that determinable that it appears to instantiate. For
example, an object might appear to fall within a certain range of
colors, while there is no exact shade of color that it appears to
have. Chisholm (1942) discusses a case in which one sees a speckled
hen for a moment, but one is unable to say how many speckles one saw.
Ayer (1963, pp. 124–5) implies that in such a case, there is no
definite number of speckles that the sense datum had. Other, perhaps
more convincing pieces of evidence for indeterminate appearances
include our inability to say exactly how far away certain objects
seem to be, our inability in some cases to say merely on the basis of
appearances whether two objects are the same color, and our inability
to read blurred or far-away words. Hardin (1985) discusses
psychological experiments that seem to demonstrate indeterminacy of
color and shape appearances: in some cases, subjects can visually
detect the existence of an object without being able to make out any
apparent color, can detect motion without awareness of the shape or
color of the moving object, and so on.

If the apparent properties of objects of perception are sometimes
indeterminate, then the sense data involved would have to be
metaphysically indeterminate — that is, they would have to
actually lack definite characteristics. This, however, is logically
impossible — an object cannot be speckled but have no
particular number of spots; an object cannot be colored but have no
particular shade of color; and so on. This sort of problem only
arises when, as sense data theorists do, one analyzes appearance in
such a way that there must always be an actual object that has all
and only the properties that appear to the subject (Huemer 2001, pp.
168–73; Armstrong 1993, pp. 218–21).

A related problem is raised by cases of inconsistent appearances, as
in the case of the waterfall illusion. This is an illusion in which
objects appear, at each moment during an extended time interval, to
be moving, yet they never change their positions in the visual field.
The sense data theory would seem to demand sense data with
inconsistent properties in such a case (Hardin 1985, p. 489).

Sense data theorists may respond to these problems by denying, pace
Ayer, that sense data have exactly the properties they appear to
have. It is unclear how much of the original motivation for
introducing “sense data” remains after the idea has been
thus revised.

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